-Although it is currently considered "high risk," we've only been
watching it for half an orbit, so more observations are needed.

Scientists are keeping a close eye on a big asteroid that may pose an
impact threat to Earth in a few decades. The space rock, which is called
2011 AG5, is about 460 feet (140 meters) wide. It may come close enough
to Earth in 2040 that some researchers are calling for a discussion
about how to deflect it.

Talk about the asteroid was on the
agenda during the 49th session of the Scientific and Technical
Subcommittee of the United Nations Committee on the Peaceful Uses of
Outer Space (COPUOS), held earlier this month in Vienna.

A UN
Action Team on near-Earth objects (NEOs) noted the asteroid's repeat
approaches to Earth and the possibility -- however remote -- that 2011
AG5 might smack into our planet 28 years from now.

The object
was discovered in January 2011 by Mount Lemmon Survey observers in
Tucson, Ariz. While scientists have a good bead on the space rock's
size, its mass and compositional makeup are unknown at present.

An Asteroid Desktop Exercise

"2011 AG5 is the object which currently has the highest chance of
impacting the Earth ... in 2040. However, we have only observed it for
about half an orbit, thus the confidence in these calculations is still
not very high," said Detlef Koschny of the European Space Agency's Solar
System Missions Division in Noordwijk, The Netherlands.

"In
our Action Team 14 discussions, we thus concluded that it not
necessarily can be called a 'real' threat. To do that, ideally, we
should have at least one, if not two, full orbits observed," Koschny
told SPACE.com.

Koschny added that the Action Team did
recommend to the NEO Working Group of COPUOS to use 2011 AG5 as a
"desktop exercise" and link ongoing studies to the asteroid.

"We are currently also in the process of making institutions like the
European Southern Observatory aware of this object," Koschny said. "We
hope to make the point that this object deserves the allocation of some
special telescope time."

Non-Zero Impact Probability

The near-Earth asteroid 2011 AG5 currently has an impact probability of 1
in 625 for Feb. 5, 2040, said Donald Yeomans, head of the Near-Earth
Object Observations Program at NASA's Jet Propulsion Laboratory in
Pasadena, Calif.

This impact probability isn't set in stone,
however. So far, researchers have been able to watch the asteroid for
just a short time -- the first nine months of 2011 -- and the numbers
may change after further observation, Yeomans told SPACE.com.

"Fortunately, this object will be observable from the ground in the
2013-2016 interval," Yeomans said. In the very unlikely scenario that
its impact probability does not significantly decrease after processing
these additional observations, "there would be time to mount a
deflection mission to alter its course before the 2023 keyhole," he
added.

Keyholes are small regions in space near Earth through
which a passing NEO's orbit may be perturbed due to gravitational
effects, possibly placing it onto a path that would impact Earth.